Components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C.
Some components of high-temperature mechanical systems include a Ni or Co-based superalloy substrate. In an attempt to reduce the temperatures experienced by the substrate, the substrate can be coated with a thermal barrier coating (TBC). The TBC may include a thermally insulative ceramic topcoat and is bonded to the substrate by an underlying metallic bond coat. The TBC, usually applied either by air plasma spraying or electron beam physical vapor deposition, is most often a layer of yttria-stabilized zirconia (YSZ) with a thickness of about 100 micrometers (μm) to about 500 μm. The properties of YSZ include low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The YSZ TBC is also typically made “strain tolerant” and the thermal conductivity further lowered by depositing a structure that contains numerous pores and/or pathways.
Economic and environmental concerns, i.e., the desire for improved efficiency and reduced emissions, continue to drive the development of advanced gas turbine engines with higher inlet temperatures. Some components of high-temperature mechanical systems include a ceramic or ceramic matrix composite (CMC) substrate, which may allow an increased operating temperature compared to a component with a superalloy substrate. The ceramic or CMC substrate can be coated with an environmental barrier coating (EBC) to reduce exposure of a surface of the substrate to environmental species, such as water vapor or oxygen. In some embodiments, the EBC also may provide some thermal insulation to the ceramic or CMC substrate. The EBC may include a ceramic topcoat, and may be bonded to the substrate by an underlying metallic or ceramic bond coat.